DISLOCATIONS ON THE WESTERN EDGE OF THE SOUTH WELSH COAL-FIELD. 165 



east and the parallel of Llandeilo Fawr on the west, a distance of about twelve miles. 

 On referring to the map it will be seen, that while the lower members of the Old Red 

 Sandstone are nearly vertical, and preserve the same rectilinear strike (north-east to 

 south-west) as the parallel ridges of Silurian rocks which trend from Shropshire to Caer- 

 marthenshire ; the upper members of the same system, which are slightly inclined, 

 constitute lofty mountains, and have, on the whole, a curvilinear direction, conform- 

 ing to the margin of the coal basin. 



The dislocations I shall describe, are two which occur near the Caermarthen Fans, 

 and one at Castell Cerrig Cennen. 



The western end of that portion of the escarpment of the South Welsh coal-field, 

 which is occupied by the Caermarthen Fans, exhibits a very powerful transverse fault, 

 by which the upper strata of the Old Red Sandstone have been so thrown up, as to 

 occupy the summit of Fan-sirgaer, 2200 feet above the sea ; while in the contiguous 

 mountain of Carreg-ogof, the same beds, covered by carboniferous limestone and mill- 

 stone grit, lie at the height of not more than 1 500 feet ; and hence the Old Red Sand- 

 stone of the Fans is proved to have been upcast to an extent of at least 700 feet 1 . The 

 direction of this transverse fault is nearly from north to south, as marked by the fissure 

 in which the rivulet Twrch-fechan flows, in its descent from the edge of the escarpment 

 towards the centre of the coal basin. On the left bank of this rivulet all is Old Red 

 Sandstone, while a little to the right is the millstone grit of Carreg-las, the limestone 

 being entirely lost for the space of nearly two miles. The elevated mass of Old Red 

 Sandstone dips ten to twelve degrees S.S.E. the limestone and millstone grit being in- 

 clined twenty degrees to the south. This inclination of the limestone accounts for its 

 rapid disappearance beneath the millstone grit of Carreg-las, and connected with the 

 great upcast of the fans, explains how the latter rock has been thrown into juxta- 

 position with the Old Red Sandstone. 



Immediately to the west of the great limeworks of Cloganmawr, there is another con- 

 siderable fault. (PI. 30. fig. 12.) After occupying the terrace- formed ridge, on which 

 the kilns are situated, the limestone is abruptly snapped off, and thrown down about 

 200 feet beneath its usual level into a morass, the ends of the lower strata only being 

 visible on the side of the mountain rill. This is the spot above Blaen-Cennen, where 

 the limestone presents the peculiar glazed surface before described. (See p. 156.) 

 The strike of this dislocated mass is 10° to the north of west, and the inclination, differ- 

 ing from that of the adjoining lime quarries, is 25° south by west. This downcast, 

 though not exhibiting so great an amount of disturbance as that of the upcast of the Old 

 Red Sandstone in Fan-sirgaer, exhibits in one respect analogous phenomena. In both 

 cases large portions of the lip of the coal-field present the appearance of having been 



1 The adjoining portion of the edge of the basin to the east of this tract has been examined by Mr. Cony- 

 beare and Mr. Maclauchlan, who have favoured me by colouring the quarter sheet of the Ordnance map in 

 which Merthyr Tidvil is situated. 



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